1. Field of the Invention
The invention relates to a flameless heater utilizing an exothermic chemical reaction to produce heat and a method of making the heater. More particularly, the invention relates to a flameless heater particularly suited for heating military field rations as well as humanitarian and recreational rations and which meets or exceeds the current U.S. military performance specification, MIL-R-44398B.
2. State of the Art
The U.S. Military has provided individual field rations in a form known as "Meal-Ready-to-Eat" ("MRE"). An MRE is a food product contained within a hermetically sealed pouch. The MRE is heated in the field with a device militarily referred to as a "Flameless Ration Heater" ("FRH"). The FRH contains as its active ingredient a supercorroding alloy comprised of magnesium 5 atomic % iron. This supercorroding alloy is blended together with ultrahigh molecular weight ("UHMW") polyethylene powder, fillers, and electrolyte, the powder mixture being placed in a mold and the mixture heated to sinter it. After cooling, the mixture forms a strong flexible pad with sufficient porosity to allow water to penetrate and wet the alloy. In order to provide the pad with sufficient strength for handling, it has been found that the pad should contain about 50% UHMW polyethylene powder. The pad is packaged within a paperboard cover having holes die cut through each side and then is hermetically sealed within a high density polyethylene bag of sufficient size to hold the heater and a filled MRE pouch. To use the heater, the top of the bag is opened and water added until the water level reaches preprinted fill lines on the bag. The water passes through the holes in the pad cover and by wicking through the paperboard, wets the pad and initiates an exothermic chemical reaction. The reaction takes the general form of: Mg+2H.sub.2 O.fwdarw.Mg(OH).sub.2 +H.sub.2 +heat (and steam).
A typical sintered pad is approximately 3.5".times.4.5".times.0.125" and weighs approximately 21 grams. When the pad is placed within the paperboard cover, the overall thickness increases to approximately 0.170". The paperboard cover adds approximately 9 grams to the overall weight of the heater, bringing the total weight of the heater to approximately 30 grams. When activated with 45-65 milliliters of water, the heater generates enough heat to raise the temperature of an 8 ounce food package 100.degree. F. above its starting temperature within 12 minutes. This performance is required by Military Specification MIL-R-44398B (Sep. 20, 1993). The components of the state-of-the-art FRH and the sintering method for forming a flexible structure of those ingredients are described generally in U.S. Pat. Nos. 4,522,190 to Kuhn et al. and 4,264,362 to Sergev et al., the complete disclosures of which are hereby incorporated by reference herein.
It is recognized that the weight of the FRH is an important consideration. It is always desirable to reduce the total amount of weight and volume which must be carried by a soldier or camper in the field, or by support elements. Another important consideration is the amount of water needed to activate the heater. Water is often scarce in the field and every ounce of water in a soldier's canteen is precious under field conditions. It is therefore generally understood that the performance of an FRH is improved by reducing its weight and volume, and by reducing the amount of water needed to activate it. It is also important to note that virtually millions of FRH units are currently packed and shipped by contractors and the military every month. Therefore, even minor reductions in the overall weight and volume of an FRH can effect a substantial reduction in shipping weight and volume and thereby reduce shipping costs as well as storage space and costs.
U.S. Pat. No. 5,117,809 to Scaringe et al., the complete disclosure of which is hereby incorporated by reference herein, discloses an improved FRH and method of making it which avoids the need for sintering by containing the loose ingredients within the confines of a cover, at least one face of which is rigid, or within a die-cut rigid skeleton. In lieu of the paperboard cover, the heater pad cover is made of a molded rigid polymeric bottom layer and a porous non-woven polypropylene top layer. The rigid bottom member provides the rigidity equivalent to '190's sintering and the porous top layer allows water to wet the powder and initiate the reaction. Alternative embodiments use a cardboard bottom layer or porous bottom layer with a rigid middle layer.
Not every military field operation requires the use of MREs and the flameless reaction heater. As mentioned above, these rations are individually packaged and designed for use in situations where more conventional heaters cannot be used. Moreover, it is often the case that group meals are prepared in the field by one or two soldiers to feed anywhere from 20 to 200 soldiers. Group meals are more efficient than individual meals because only a few soldiers need to be distracted from other duties for the preparation of the meal. Recently, it has been proposed that the concepts of the MRE be applied to group meals. The proposed "Self-Heating Group Meal" ("SHGM") is described in U.S. Pat. No. 5,355,869, the complete disclosure of which is hereby incorporated by reference herein. The SHGM described in the '869 patent utilizes a number of heating trays which are provided with stand-offs on their bottom surfaces and a corresponding number of FRH packages which are supported in the trays by the stand-offs. Food pouches containing 6.6 pounds of food are placed directly on top of the FRH packages and water is delivered into the space, created by the stand-offs, between the trays and the FRH packages. The trays are typically 13".times.10".times.1.5" deep and use slightly modified versions of the FRH packages (12.75".times.9.75".times.0.156") described in the above-cited '190 and '809 patents.